GDM, the GNOME Display Manager, handles authentication-related backend
functionality for logging in a user and unlocking the user's session after
it's been locked. GDM also provides functionality for initiating user-switching,
so more than one user can be logged in at the same time. It handles
graphical session registration with the system for both local and remote
sessions (in the latter case, via the XDMCP protocol). In cases where the
session doesn't provide it's own display server, GDM can start the display
server on behalf of the session.